Microsoft is attacking Google with a marketing campaign focused on a recent change in how Google runs the part of its search engine devoted to shopping results.

The marketing campaign, lauched by Microsoft and its search engine Bing, has been designed to "highlight Bing's commitment to honest search results and to help explain to consumers the risks of Google Shopping's newly announced "pay-to-rank" practice." According to Microsoft, Google's shopping search results are offering online visitors not true search results such as they see elsewhere on Google; they are actually ads that are ranked, in part, by who pays the most. Microsoft claims that merchants can literally pay to improve their chances to display their product offers higher than others inside of Google?s shopping search, even if it's not necessarily better or cheaper.

Since mid-October, Google's shopping section has included only listings from merchants who paid to be included in the results. In some cases, the order of the shopping results has been dictated by how much money Google received for the listing.

Google discloses that it receives payments in small print at the bottom of the shopping results page. The notice is also visible if a user clicks on a link at the top of the shopping results page, under the heading: "Why these products?"

In its new ads running in the U.S., Microsoft is warning consumers that they risk getting "scroogled" if they rely on Google's shopping search service. The message will be highlighted in TV commercials scheduled to run on NBC and CNN and newspaper ads in The New York Times, The Wall Street Journal and The Washington Post. The blitz also will appear on billboards and online, anchored by a new website, Scroogled.com.

On the other hand, Bing, affirms its commitment to honest search. "We don't let who pays us for ads or other services affect how your search results are ranked," said Mike Nichols, chief marketing officer, Bing. "Search, as a business, depends on consumer trust, and that requires keeping search results and ads separate. With Google Shopping the wall between search results and ads is gone - and so are several popular shopping sites. At Bing, we're committed to keeping ads where they belong and will continue to deliver the most relevant search results possible."